1. Field of the Invention
The invention relates to a knock control for an internal combustion engine having a variable valve mechanism.
2. Description of the Related Art
Conventionally, various arts related to knock control for gasoline engines have been proposed. Among them, retarding an ignition timing is known as a typical control for controlling knocking. Japanese Patent Laid-opened Publication No. 11-324762 discloses an art that retards a timing of closing intake valve when a knocking occurs. Furthermore, Japanese Patent Laid-opened Publication No. 11-190236 discloses an art that, when a knocking occurs, retards or advances the phase of intake valve-open period according to the state of the knocking. Moreover, an art that changes a timing of closing an intake valve according to the degree of knocking is disclosed in Japanese Patent Laid-Open Publication No. 06-129271.
Meanwhile, there is known, as a mechanism of an intake valve, a so-called variable valve mechanism that is capable of changing both length of a valve-open period (i.e., operating angle) and a position of the valve-open period (i.e., phase of valve-open period). With such a variable valve mechanism, the volume of intake air can be reduced by reducing the operating angle, instead of narrowing down the opening of a throttle valve of an intake system, during a low-load running of the engine. As a result, a resistance of the intake system is reduced, and thus, a pump loss is reduced, thereby increasing thermal efficiency.
In fact, there have not been many improvements for a knock control in an internal combustion engine having a variable valve mechanism as described above. The operating angle of an intake valve of an ordinary internal combustion engine is usually set at 220 to 240 degrees, whereas the operating angle of an intake valve of an internal combustion engine having a variable valve mechanism may in some cases be set smaller than 180 degrees. When the operating angle of the intake valve is smaller than 180 degrees, a knock control effect obtained by advancing and retarding the phase of valve-open period significantly differs from that obtained in the case of the operating angle of 200 degrees or larger. However, as a matter of fact, not enough countermeasures have been devised against knocking in an internal combustion engine in which the operating angle may be set at an angle that is smaller than 180 degrees.